feat(VSN-2494): Salarie/RH/Recrutement CSS grid migration, circuit Modifier fix, and UI component refinements

Summary

UI Components

• Calendar: Fixed month/year dropdown z-index (z-[1600]) so it renders correctly above outer popovers in form fiche fields
• Form-field: Refined combobox and multi-select hover states to use soft purple (#D4CAFE) instead of solid primary
• Tabs-custom: Minor tab hover style cleanup

Circuit

• Modifier button fix: Replaced nested <form> with <div> inside circuit-form.tsx to eliminate invalid HTML nesting that was silently swallowing click events. Button is now rendered at the top of the Circuit tab panel, below the tab navigation.

Salarie

• Full migration of /dashboard/rh/salarie from MUI TableManager to CSS grid table pattern (matching the canonical circuit-table.tsx pattern)
• New salarie-table.tsx component with 3-way synchronized scroll (header / body / horizontal scrollbar), skeleton loaders, empty state, and backend pagination props
• salarie-list-view.tsx rewritten: removed all MUI imports, wired to useSalarieBackend hook with full sort/filter/pagination mapping

RH — Formulaire & Recrutement

• formulaire-table, formulaire-list-view, formulaire-table-row migrated to CSS grid layout with shadcn components
• besoin-recrutement-table, besoin-recrutement-list-view, besoin-recrutement-table-row migrated
• candidature-table, candidature-list-view, candidature-table-row, candidature-formulaire, formulaire-table-row migrated

Chauffeur — Planning Trajet

• Minor refactor of details-planning-trajet.tsx and sortable-planning-trajet-row.tsx

Closes VSN-2494